Motor drive system for rolling mill

ABSTRACT

In a four high rolling mill having two powered working rolls and a pair of backup rolls, the backup rolls are synchronized by controlling the drive motors to force differential slippage between the working rolls and the strip being processed through the mill. Angular position comparison means senses and lack of synchronism of the backup rolls and produces a control signal which differentially adjusts the torques produced by the drive motors. Synchronization of the backup rolls reduces roll force variations caused by eccentricity of rolls.

United States Patent.

Cox

[541 MOTOR DRIVE SYSTEM FOR ROLLING MILL [72] Inventor: Howard NesbitCox, Boxford, Mass.

[73] Assignee: General Electric Company [22] Filed: Feb. 22, 1971 211App]. No.: 117,319

[52] US. Cl. ..72/8, 72/21, 72/29 [51] Int. Cl. ..B2lb 37/00 [58] Fieldof Search ..72/8, 19, 2], 29, 249

[56] I References Cited UNITED STATES PATENTS 3,298,212 1/1967 Cook..72/8 3,331,229 7/1967 Neumann et a1. ..72/8

[ 1 Aug. 15, 1972 Primary Examiner-Milton S. Mehr Attorney-James C.Davis, John J. Kissane, Frank L. Neuhauser, Oscar B. Waddell and JosephB. Forman [5 7] ABSTRACT In a four high rolling mill having two poweredworking rolls and a pair of backup rolls, the backup rolls aresynchronized by controlling the drive motors to force differentialslippage between the working rolls and the strip being processed throughthe mill. Angular position comparison means senses and lack ofsynchronism of the backup rolls and produces a control signal whichdifferentially adjusts the torques produced by the drive motors.Synchronization of the backup rolls reduces roll force variations causedby eccentricity of rolls.

10 Claims, 4 Drawing Figures Patented Aug. 15, 1972 2 Sheets-Sheet 1 mm"IIII FIG. I

T|ME- FIG. 2

INVENTOR HOWARD N. COX

Patented Aug. 15, 1972 2 Sheets-Sheet 2 SEL SYN I I I I I I I I l I I II I I I l I I I I I I I I I I I I I l I I I I I l I I l I l I I I I I ILXI I l l INVENTOR HOWARD N. COX

FIG. 4

BACKGROUND OF THE INVENTION This invention relates to a motor drivesystem for rolling mills of the type used to reduce the thickness ofstrip material passing through the mill.

Rolling mills of the so-called four high type are widely used.Typically, the mill comprises a pair of juxtaposed working rolls betweenwhich the strip to be processed passes, the working rolls being drivenin opposite directions by powerful electric motors coupled to the rollsthrough a gear reduction drive system. To resist the large forcestending to separate the working rolls massive backup rolls are rotatablymounted in a mill frame so as to bear against the working rolls onopposite sides of the work rolls where the strip reduction takes place.The backup rolls are forced into engagement with the working rolls byscrew-down apparatus and rotate with the working rolls.

A major problem in successful operation of a rolling mill is to maintainthe processed strip material at a relatively constant thickness or gage.One cause of thickness variation is eccentricity of the backup rollswhich results in roll force variations with corresponding changes instrip thickness. This roll force variation is random in nature becauseof lack of synchronism of the backup rolls during operation of themill.This lack of synchronism is caused by differential slippage between theworking rolls and the processed strip. It is also caused by the backuprolls having slightly different diameters resulting from regrinding ofthe rolls made necessary by wear. Because of the random nature of theroll force variation caused by roll eccentricity it is difiicult todetect and control. Attempts have been made to compensate for rolleccentricity by continuous sensing of the roll force and adjustment ofthe screwdown apparatus to compensate for roll force variations.However, such control systems are costly to build and expensive tomaintain.

Accordingly, it is an object of this invention to provide an improvedsystem for driving and controlling a rolling mill that reduces andessentially eliminate roll force variations and resulting thicknessvariations of the processed strip caused by eccentricity of the backuprolls.

Another object of the invention is to provide a drive system for arolling mill which reduces the effect of roll eccentricity byautomatically and continuously synchronizing the backup rolls during theoperation of the mill.

Further objects and advantages of the invention will become apparent asthe following description proceeds.

SUMMARY In accordance with the invention the backup rolls of a rollingmill are continuously synchronized by differentially adjusting thetorques applied by the drive motors to the working rolls. This producesa differential slippage between the work rolls and the processed stripin a direction to synchronize the backup rolls. A position comparatorcontinuously compares the instantaneous angular positions of the backuprolls and produces an output signal indicative of any 2 departure of theback rolls from a predetermined relative angular position that producesminimum roll force variation due to eccentricity of the backup rolls.The comparator output signal differentially adjusts the drive motortorques in any suitable manner as by coaction with the motor fieldcurrent regulators. Position comparison of the backup rolls is performedby selsyns or other known types of angular position comparison devices.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic illustration ofa rolling mill to which the drive system of the present invention may beapplied.

FIG. 2 is a graphical representation showing the manner in which rollforce variation occurs in the mill of FIG. 1 due to eccentricity of thebackup rolls.

FIG. 3 is a geometrical diagram illustrating how slippage occurs betweenthe rolled strip and the working rolls of a mill, and

FIG. 4 shows, in schematic fonn, a motor drive and control system forthe mill of FIG. lwhich embodies the present invention.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT Referring now to FIG. 1 of thedrawing there is illustrated in schematic form a 4 high rolling mill towhich a motor drive system embodying the present invention may beapplied. As shown the mill comprises a pair of juxtaposed working rolls10 and 11 having therebetween an interface 12 through which a strip 13to be rolled passes. The working rolls 10 and 11 are separately drivenin opposite directions by electric drive motors l4 and 15 throughsuitable gear reductions 16 and 17.

Bearing against the working rolls on opposite sides of the interface 12are upper and lower backup rolls l8 and 19 the journals of which aresupported in bearing blocks 20 and 21. Downward pressure exerted onupper backup roll 18 is adjusted by screw-down apparatus comprising athreaded screw 22 in the threaded engagement with a stationary framemember 23. The screw 22 terminates in a gear 24 driven by a reversiblescrewdown motor (not shown) providing a means for adjusting rollingforce exerted by the mill.

The lower bearing block 21 is supported on a base 25 through asupporting member 26. Interposed between support 26 and base 25 is aload cell 27 of known construction providing a means for measuring millrolling force on a suitable instrument 28.

The journals of the working rolls l0 and 11 are supported in bearingblocks 29 and 30 which are slidably supported to permit limited verticaladjustment of the working rolls. The bearing blocks 20 and 21 of thebackup rolls are relatively fixed and because of this any eccentricityin the backup rolls causes substantial variation in the rolling force.For example, a relative vertical movement of the contact points P and Iwhere the backup rolls engage the working rolls of 0.001 inch will causea change in the roll force of 15 tons in a typical mill.

The backup rolls 18 and 19 are usually slightly eccentric due tomanufacturing inaccuracies. Eccentricity may also be caused by bendingof the roll journals due to excessive transient roll force. If the highpoints on the backup rolls reach the contact points P and Psimultaneously the roll force will be a maximum. After 180 of rotationthe low points on the backup roll reach the contact points P and Psimultaneously and the roll force will be at a minimum value. For thiscondition the difierence between the maximum and minimum roll forces fora complete revolution of the backup rolls will be at a maximum. On theother hand if the angular relation of the backup rolls is such that thehigh point of one roll reaches one of the contact points at the sametime that the low point on the other roll reaches the other contactpoint the difference between the maximum and minimum roll forces for acomplete revolution of the backup rolls will be a minimum. Forconditions where the angular relationships of the backup rolls arebetween the maximum and minimum conditions described above, thedifference between the maximum and minimum roll forces during a completerevolution of the backup rolls will have some intermediate value.

During operation of a conventional four high rolling mill of the typeshown in FIG. 1 the upper and lower backup rolls are likely to rotate atdifferent speeds because the backup rolls have different diameters. Thiscondition may, for example, be the result of different amounts ofregrinding of the rolls to remove surface irregularities. Differentspeeds and angular relationships of the upper and lower backup rolls mayalso be caused by differential slippage between the working rolls andthe strip being rolled. Because of the speed differential and slippagethe angular relationship of the backup rolls continuously changes. As aresult of the lack of synchronization of the backup rolls the roll forcevariations occurring during each rotation of the backup rollscontinuously changes in a random and unpredictable manner.

Referring to FIG. 2 the roll force variation caused by eccentricity andcontinuously varying angular relationships of the backup rolls isgraphically illustrated. Such roll force variation is indicated, forexample, by the reading of instrument 28 at different times T. Duringthe time interval T,, T the roll force variation will have a maximumoscillation or range F F which occurs when the high points of the backuprolls contact the working rolls at the same time. As the relativeangular positions of the backup rolls change due to slippage at the workroll interface or because of different backup roll diameters an intervalT T will be reached when the roll force oscillation will have a minimumvalue F F This occurs when the high point of one backup roll and the lowpoint of the other backup roll contact the working rolls at the sametime. As the relative angular positions of the backup rolls continue tochange the roll force will reach another maximum oscillation during thetime interval T T The dotted lines V forming the envelope of roll forcecurve F show the cyclic manner in which the amplitude of the roll forceoscillations varies as the relative angular position of the backup rollscontinuously changes due to lack of synchronism of the rolls duringoperation of the mill. In actual practice the number of oscillations ofthe roll force curve F between maximum and minimum points will likely beconsiderably greater than the number shown which has been reduced forclarity and ease of illustration.

According to the invention the backup rolls l8 and 19 are synchronizedand locked in a relative angular position which produces a minimum rollforce variation caused by eccentricity of the backup rolls. Thisminimizes changes in thickness or gage of the rolled strip caused bysuch roll force variations resulting in a better product and less scraploss of off-gage material exceeding permissible thickness variations.This is accomplished in a manner now to be described by causingdifferential slippage to occur between the work rolls and the rolledstrip during operation of the mill.

The invention makes use of the fact that slippage necessarily occursbetween the working rolls l0 and 11 and the rolled strip 13. The natureof this slippage is illustrated in FIG. 3. As the strip 13 passesthrough the working rolls 10 and 1 1 its thickness is reduced from H toH Because of extrusion effects accompanying the thickness reduction thevelocity V of the strip after it passes through the rolls is necessarilygreater than the entering velocity V for constant mass flow through themill. At neutral points N on the interface of the working rolls theperipheral velocity of the rolls equals the strip velocity after leavingthe mill. At points of contact between the strip and the rolls beyondpoints N the strip velocity exceeds the peripheral velocity of the rollsso that there is slippage therebetween. At points of contact ahead ofpoints N the strip velocity is less than the peripheral velocity of therolls so that there is slippage therebetween in the reverse direction.Thus slippage continuously occurs between the working rolls and therolled strip. According to the invention, this slippage isdifferentially adjusted by varying the torques exerted on the workingrolls by their associated drive motors. This differential slippage ismade to occur in a direction to restore the backup rolls to thesynchronized position producing minimum roll force variation in responseto a departure of the backup rolls from that position. This automaticcontrolling action is incorporated into the motor drive system of theworking rolls. An illustrative way in which this control action may beaccomplished will now be described.

Referring now to FIG. 4, the drive motors 14 and 15 for the workingrolls 10 and 11 are shown as DC. motors having armatures energized froma common bus 31 connected by a circuit breaker 32 to a DC. power supplysuch as a generator 33.

The motors 14 and 15 have field windings 34 and 35 supplied with directcurrent by associated field current regulators 36 and 37. The fieldcurrent regulators, which are shown schematically, are self-saturatingmagnetic amplifiers commonly referred to as amplistats. Such amplifiersare shown, for example, in US. Pat. No. 3,132,293 Marrs, issued May 5,1964, to which reference may be made for construction details. Theamplifier-regulators are energized from A.C. power sources 38 and 39 andhave DC. output circuits 40 and 41 in which the output current iscontrolled in accordance with the net D.C. magnetic control fluxsupplied by control windings. As shown, the regulator 36 has threecontrol windings 42, 43 and 44 and the regulator 37 has three controlwindings 45, 46 and 47. The motor field windings 34 and 35 are connectedto the output circuits 40 and 41 of regulators 36-and 37 and the DC.current supplied to these field windings is controlled by adjustment ofDC. current supplied to the control windings. To pemiit simultaneousadjustment of the field currents of the motors l4 and the controlwindings 43 and 46 are connected in series, as shown, and energized fromthe output of a rheostat 48. The rheostat is driven betweenpredetermined stable operating limits by a reversible motor 49 which ismanually operated by suitable controls (not shown) to adjust theoperating speeds of the working rolls l0 and 1 1 The control windings 42and 45 of the field current regulators are energized in accordance withthe output currents in their output circuits, as shown, and in thismanner a feedback action occurs by which the motor field currents aremaintained at values preset by the outputs of the other controlwindings. The control windings 44 and 47 are provided to permit adifferential adjustment of the field currents and hence torque outputsof the drive motors 14 and 15 by means of which the backup rolls l8 and19 are continuously synchronized as will be more fully described.

During operation of the mill any lack of synchronism of the backup rollsis detected by angular comparator apparatus which, in the formillustrated, is a differential selsyn system. The system comprises twoselsyns 50 and 51 and a differential selsyn 52. Selsyn 50 has a rotor 53mechanically coupled to the journal of the upper backup roll 18, theconnection being indicated by the dash line 55. Similarly, the selsyn 51has a rotor 54 mechanically coupled to the journal of the lower backuproll 19, the connection being indicated by the dash line 56. The selsyns50 and 51 have rotor windings 57 and 58 and stator windings 59 and 60,the rotor windings being energized from a common AC. power source 61.With this arrangement, the electrical outputs of the selsyn statorwindings-are indicative of the angular positions of the back rolls 18and 19.

The differential selsyn 52 has a rotor 62 with a rotor winding 63 and astator winding 64. As show, the stator winding 59 of selsyn 50 isconnected to the stator winding 64 of the differential selsyn, and thestator winding 60 of selsyn 51 is connected to the rotor winding 63 ofthe differential selsyn. The connections and polarities are chosen sothat when the backup rolls are driven by the motors l4 and 15 throughthe working rolls 10 and 1 1, the magnetic fields in the rotor andstator windings of the differential selsyn rotate in the same direction,assumed for purposes of explanation to be clockwise. Thus, when thebackup rolls rotate at the same speed the rotor 62 of the differentialselsyn remains stationary. However, if the backup rolls begin to rotateat different speeds the rotor 62 of the differential selsyn will rotatein a direction dependent on whether one of the backup rolls leads orlags the other backup roll. Thus, the mechanical output of thedifferential selsyn rotor provides an output signal indicative of lackof synchronism of the backup rolls during operation of the mill. Bycoupling the output of the differential system to the differential fieldcurrent control windings 44 and 47 of the regulators 36 and 37 the drivemotor torques and the slippage between the working rolls 10 and 11 andthe rolled strip 13 are controlled so as to correct continuously anylack of synchronism of the backup rolls. The coupling system by whichthis is accomplished will now be described.

The mechanical output of the differential selsyn 52 is first convertedto a DC. control signal the magnitude and polarity of which isindicativeof the'direction and amount of displacement of thedifferential selsyn rotor from a null position. For this purpose thereis provided a potentiometer 65 comprising a fixed resistance ele ment 66and a rotatable wiper 67. The ends of the resistance 66 are connected toplus and minus terminals of a suitable D.C. power supply with a midpoint68 grounded so as to be at zero potential. The wiper 67 is biased by aspring 69 so that it normally occupies the zero or null output positionshown but can be forcibly displaced in either direction to produce plusand minus output signals. These signals are fed by lead 70 to the inputof an amplifier 71 the output of which is connected by lead 72 to theserially connected differential control windings 44 and 47 of the fieldcurrent regulators 36 and 37. The amplifier input and output circuitsare bridged by a capacitor 73 so that the amplifier perfonns anintegrating function. With this arrangement the amplifier DC. output andthe current supplied to windings 44 and 47 is increased or decreaseddepending on the direction of displacement of potentiometer wiper 67from the null position. Also, the rate of change of amplifier outputvaries with the degree of displacement of the potentiometer wiper fromthe null position. It will be noted that the polarities of windings 44and 47 are chosen so that current flowing therethrough produces oppositeeffects on the current output of field current regulators 36 and 37.Thus a current flowing in circuit 72 from the amplifier in one directionwill increase the output of field current regulator 36 and decrease theoutput of regulator 37 and vice versa. Since the torque produced by thedrive motors 14 and 15 depends on the field currents supplied thereto inthe normal motor operating speed range, it will now be clear that bycontrolling the direction and magnitude of the current in circuit 72 themotor torques may be differentially adjusted in either direction and tothe desired amount, this action being accomplished by movement of thewiper 67 of potentiometer 65 in either direction from the null position.

To complete the control system loop the potentiometer wiper 67 iscoupled to the rotor 62 of the differential selsyn 52 through a clutch74 the interconnection being represented by dash lines 75 and 76. Theclutch 74 may be electrically operated to permit engagement by closureof a control switch 77. With the clutch disengaged the differentialselsyn rotor 62 is free to rotate and biasing spring 69 maintainspotentiometer wiper 67 at the null position. When the clutch is engagedrotor 62 will drive wiper 67 off null in either direction to effect adifferential adjustment of the output torques of the drive motors 14 and15. Under certain operating conditions, the excursions of the controlsystem and the motor speeds may exceed normal operating limits. Undersuch conditions, it may be desirable momentarily to zero the output ofamplifier 71. For this purpose a shunting circuit controlled by a switch78 is provided. The switch 78 may be operated manually or automatically.

OPERATION producing minimum roll force variation may now be described.

Initially, switch 77 is opened to disable the automatic motordifferential field control. Backup rolls 18 and 19 are then positionedto have the relative angular relationship which produces minimum rollforce variation caused by eccentricity of the backup rolls. One way todetermine this predetermined relative angular position is to energizedrive motors 14 and 15 as by closing circuit breaker 32 to rotate themill rolls. Because the backup rolls usually have different diametersthe angular relationship between the backup rolls will continuouslychange. By observing instrument 28 the point of minimum roll forcevariation can be determined at which point switch 77 is closed toactivate the differential field control system. Use of a recording typeof instrument facilitates this operation by producing a roll forceamplitude curve similar to that shown in FIG. 2. After switch 77 isclosed the drive system automatically acts to maintain synchronism ofthe backup rolls in the desired relative angular relationship in thefollowing manner.

Referring to FIG. 4, it will be assumed for purposes of furtherexplanation that the upper backup roll 18 has a high spot l-I whoseinstantaneous angular position with reference to a fixed point islocated at angle (9 and that the lower backup roll 19 has a low point Llocated at an angle as shown on the drawing. If angles 0 and 0 are equaland the backup roll speeds are equal the high and low points H and Lwill reach the points of contact with the working rolls at the sameinstant. This, then, is the desired angular relationship between thebackup rolls for minimum roll force variation due to eccentricity of therolls as explained in connection with FIG. 2.

During operation of the mill this desired angular relationship will tendto change due to different backup roll diameters or differentialslippage between the working rolls and the rolled strip 13 or acombination of these two effects. For example, assume that backup roll18 moves ahead of backup roll 19 so that angle 0 Differential selsynrotor 62 and potentiometer wiper 67 will then rotate clockwise an amountequal to 0 0 Potentiometer wiper 67 is moved ofi the null position andcurrent flows through circuit 72 and windings 44 and 47 of the fieldcurrent regulators to produce a control flux in the direction of thearrows. In the case of regulator 36 this is in a direction to aid theflux produced by speed control winding 43 thereby increasing the fieldcurrent and back EMF of drive motor 14. This decreases the motor currentand output torque. On the other hand, in regulator 37 the flux producedby coil 47 opposes the flux produced by speed control winding 46 therebydecreasing the field current and back EMF of drive motor 15. Thisincreases the motor current and output torque. As a result of thesemotor torque changes in opposite directions more slipping betweenworking roll 11 and strip 13 occurs than the slippage between workingroll and strip 13. This causes backup roll 19 to rotate faster thanbackup roll 18 until the angles 0 and 0 are again equal. At this pointdifferential selsyn rotor 62 will have rotated back to its originalposition for zero output of potentiometer 65. If backup roll 19 tends tomove ahead of roll 18 angle t9 becomes larger than 0 The reverse actionthen takes place in response to counterclockwise movement ofdifierential selsyn rotor 62.

It will be noted that due to the integrating action of amplifier 71 thecontrol system will stabilize, i.e., the potentiometer 65 will be at thenull point with current flowing in circuit 72 in either direction. Thisis accomplished by combining the proportioning action of potentiometer65 with the integrating action of amplifier 71.

It will be understood that the control principles of the presentinvention may be applied to other types of motor control systems withoutdeparting from the invention. For example, the relative torque outputsof drive motors l4 and 15 may be controlled by differential adjustmentof their armature voltages rather than field currents and adjustablespeed A.C. motors may be used as will be readily apparent to thoseskilled in the art. Also, other types of know angular comparisonsystems, both analog and digital, may be used instead of the selsynsystem illustrated to control the drive motor torque differential.

The motor drive and control system embodying the present invention hasbeen illustrated as applied to a rolling mill wherein the backup rollsare rotatably driven by the work rolls. It may also be applied to a millwhere the backup rolls are directly driven by the drive motors and theworking rolls are driven by the backup rolls.

While there has been shown what is presently considered to be apreferred embodiment of the invention, it will be apparent to thoseskilled in the art that various changes and modifications may be madetherein without departing from the spirit and scope of the invention.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. A drive system for a rolling mill comprising a pair of engagingworking rolls having an interface through which a strip to be rolledpasses, and a pair of backup rolls, each backup roll engaging itsassociated working roll on opposite sides of said interface and beingrotatably driven with the working roll, said system comprising:

a. a pair of motors for driving the working and backup rolls, each motorbeing separately connected to drive its associated working and backuprolls,

b. control means for differentially adjusting the torques applied to theworking rolls by the drive motors,

c. comparator means arranged continuously to compare the instantaneousangular positions of the backup rolls and produce an output signalvariable in accordance with deviation of the relative angular positionsof the backup rolls from a predetermined position, and

(1. coupling means activating said control means in accordance with theoutput signal from said comparator means whereby the relative magnitudesof the torques applied to the working rolls are adjusted to causedifferential slip to occur between the working rolls at said interfaceto maintain the backup rolls in said predetermined relative angularposition during operation of the mill.

2. A drive system as set forth in claim 1 wherein the motors have fieldssupplied with adjustable current and the control means comprises meansfor differentially 5. A drive system as set forth in claim 3 including aclutch interposed between the mechanical output of the differentialselsyn and the control means.

6. A drive system as set forth in claim 1 wherein each drive motor has aseparate field current regulator and the control means comprises meansfor differentially adjusting the outputs of the field currentregulators.

7. A drive system as set forth in claim 6 wherein the means fordifferentially adjusting the output of the field current regulatorsincludes a circuit energized from the output of an integrating amplifiercontrolled by said comparator means.

8. A drive system as set forth in claim 7 including means for supplyingthe integrating amplifier with an input signal the magnitude andpolarity of which is variedin accordance with the direction andmagnitude of the output signal from the comparator means.

9. A drive system as set forth in claim 8 wherein the comparator meansis a selsyn system comprising a differential selsyn having rotor andstator windings energized by selsyn generators coupled to the backuprolls, the mechanical deflection of the difi'erential selsyn rotorconstituting the comparator output signal.

10. A drive system as set forth in claim 9 including a clutch interposedbetween the differential selsyn and the control means.

1. A drive system for a rolling mill comprising a pair of engagingworking rolls having an interface through which a strip to be rolledpasses, and a pair of backup rolls, each backup roll engaging itsassociated working roll on opposite sides of said interface and beingrotatably driven with the working roll, said system comprising: a. apair of motors for driving the working and backup rolls, each motorbeing separately connected to drive its associated working and backuprolls, b. control means for differentially adjusting the torques appliedto the working rolls by the drive motors, c. comparator means arrangedcontinuously to compare the instantaneous angular positions of thebackup rolls and produce an output signal variable in accordance withdeviation of the relative angular positions of the backup rolls from apredetermined position, and d. coupling means activating said controlmeans in accordance with the output signal from said comparator meanswhereby the relative magnitudes of the torques applied to the workingrolls are adjusted to cause differential slip to occur between theworking rolls at said interface to maintain the backup rolls in saidpredetermined relative angular position during operation of the mill. 2.A drive system as set forth in claim 1 wherein the motors have fieldssupplied with adjustable current and the control means comprises meansfor differentially adjusting the field currents supplied to the drivemotors.
 3. A drive system as set forth in claim 1 wherein the comparatormeans is a selsyn system comprising a differential selsyn having statorand rotor windings energized by selsyn generators coupled to the backuprolls, the mechanical deflection of the differential selsyn rotorconstituting the output signal.
 4. A drive system as set forth in claim1 including disconnect means for disabling the control means until thebackup rolls acquire said predetermined relative angular position duringrotation thereof by the drive motors.
 5. A drive system as set forth inclaim 3 including a clutch interposed between the mechanical output ofthe differential selsyn and the control means.
 6. A drive system as setforth in claim 1 wherein each drive motor has a separate field currentregulator and the control means comprises means for differentiallyadjusting the outputs of the field current regulators.
 7. A drive systemas set forth in claim 6 wherein the means for differentially adjustingthe output of the field current regulators includes a circuit energizedfrom the output of an integrating amplifier controlled by saidcomparator means.
 8. A drive system as set forth in claim 7 includingmeans for supplying the integrating amplifier with an input signal themagnitude and polarity of which is varied in accordance with thedirection and magnitude of the output signal from the comparator means.9. A drive system as set forth in claim 8 wherein the comparator meansis a selsyn system comprising a differential selsyn having rotor andstator windings energized by selsyn generators coupled to the backuprolls, the mechanical deflection of the differential selsyn rotorconstituting the comparator output signal.
 10. A drive system as setforth in claim 9 including a clutch interposed between the differentialselsyn and the control means.